High frequency amplifier



June 22, 1937. Y R. A. BRADEN 2,084,475

HIGH FREQUENCY AMPLIFIER FiledDec. 29, 1954 Fig. 1.

INVENTOH Rene ALBraden 1:30 2'00 3'00 4'00 5'00 760 1.755 a FEEOl/f/VGY//v K/LOCYCLES BY Q l ll 7' 7'0 7? N Y Patented June 22, 1937 UNITE STAHIGH FREQUENCY All'iPLIFIER of Delaware Application December 29, 1934,Serial No. 759,645

6 Claims.

My invention relates to high-frequency amplifiers, and particularly toamplifiers adapted to amplify uniformly all signal frequency componentsincluded within a wide. frequency range.

An object of my invention is to provide an improved high-frequencyamplifier having satisfactory gain at the high frequency end of the bandof frequency components to be amplified.

A further object of my invention is to provide an improved circuitwhereby the range of uniform amplification may be extended to higherfrequencies than can be done with previously known circuits.

A still further object of my invention is to 15 provide an improvedamplifier of the above mentioned type. having gain control means whichmay be adjusted without appreciably changing the amplifier frequencyresponse characteristic.

My amplifier circuit resembles the circuit shown in the U. S. patent toRound #1341383 in that an inductance coil is resonated with tube capacity at the high frequency end of the amplifiers frequency range. Inmy amplifier, how ever, the inductance coil is made to resonate 25 withthe tube output capacity rather than the tube input capacity as in theRound circuit, and the output voltage is taken off a resistor in theresonant plate circuit by means of a variable tap.

By adjusting this tap, the gain of the amplifier 30 as a whole may bevaried without changing the amplifier characteristics appreciably.

Other objects, features and advantages of my invention will appear fromthe following description taken in connection with the accompanyingdrawing, in which Figure 1 is a circuit diagram of an amplifierembodying my invention;

Fig. 2 is a circuit diagram of another embodiment of my invention; and

Fig. 3 is a group of curves showing certain characteristics of theamplifier shown in Fig. 1.

Referring to the embodiment of my invention shown in Fig. 1, theamplifier comprises an amplifier tube I which may be a vacuum tube ofthe screen grid type having a cathode 3, a control grid 5, a screen grid'1 and a plate 9. In the specific amplifier illustrated, the vacuum tubeis an RCA 224. Thecontrol grid 5 is biased to a suitable operatingpotential in any suitable manner as by means of a biasing battery II andgrid resistor IS.

A suitable positive potential is applied to the screen grid 1 through aresistor IS in the usual manner, the screen grid being maintained atsub- 5 stantially ground potential for alternating ourrents by means ofthe usual by-pass condenser ll. As indicated at C by the dotted lines,the plate 9 hasa certain amount of capacity to ground. The greater partof this capacity is the capacity between the plate 9 and the screengrid 1. As is well understood in the art, capacity shunting an amplifiercircuit in this manner tends to by-pass the higher frequency componentsof the signals to be amplified whereby the gain of the amplifier tendsto drop at the high frequency end; This is especially true where suchhigh frequencies as those employed in television systems are involved.

In accordance with my invention, I apply the usual positive potential tothe plate 9 through a plate circuit which includes an inductance coil l9positioned adjacent to the plate. The plate circuit also includes inseries with the inductance coil iii a variable damping resistor 2| foradjusting the damping of the plate circuit, and a load resistor 23across which appears the voltage to be impressed upon the succeedingamplifier tube 25.

The succeeding amplifier tube 25 may be of any suitable type, such as ascreen grid tube hav-.

ing a cathode 21, a control grid 29, a screen grid 3! and a plate 33. Asindicated at Cg by the dotted lines, the control grid 29 has a certainamount of capacity to ground which, like the capacity C has a shuntingeffect on the high frequency components of the signals. The control grid29 may be biased to a suitable operating potential in any suitablemanner as by means of the biasing battery 35 and grid resistor 31.

The output circuit of the first amplifier tube 1 is coupled to the inputcircuit of the succeeding tube 25 by means of a connection from avariable tap 39 on the resistor 23 through a coupling condenser M to thecontrol grid 29. It will'be evident that the voltage appearing acrossthe portion of the resistor 23 below the variable tap 39 is impressedupon the input circuit of the amplifier tube 25, and that the magnitudeof this impressed voltage may be changed by adjusting the variable tapwhereby the gain of the amplifier may be controlled.

As previously mentioned, the inductance coil [9 is given a value ofinductance such that it resonates with the capacity Cp at thehigh-frequency end of the amplifiers frequency range. Therefore, theplate circuit is resonant at thisselected frequency and the height ofthe resonant peak may be changed by adjusting the variable resistor 2!.This will be seen by referring to Fig. 3, where the frequency responsecharacteristic of the 7 It should be noted that the input circuit of theamplifieri'is. shown for three; values of theresistor 2;]

amplifier'tube 25 is not coupled across any portion of the platecircuitcontaining reactance.

For this reason thevoltage applied to the input circuit is directlyproportional to the current flows ,15 producedis that caused by a slightchange'in i impedance at high frequencies has no effect on the outputcurrent, which therefore remains conmg in the plate circuit'and throughthe resistor 7 2 3. Consequently, the frequency response characteristicofthe amplifier as a wholeremains substantially unchanged when theposition of the Variable tap 39 is changed. The only change that theloading of the plate circuit.

thus causing the amplification to decrease.

so high in comparison with the load impedance 23 that the abovementioned variation of the load.

stant when the frequency is varied. 7 :The efiectof the inductance IS inthe circuit can bepartly explainedby noting thatit separates C and Cg,and only Cg isdirectly across the load resistance. SinceC is generallyabout half 7 through the load impedance (comprising resistor 23 andcapacity Cg in parallel) by adding tothe output current of the tube l acirculating current component flowing through Cp, coil 19, and resistors2! and 23,.thus increasing the voltage developed aross theloadresistance 23. This'effect becomes more pronounced, of course, as thefrequency increases, until the resonanceifrequency is reached.-,Above-this frequency the eifectis reversed and the amplificationisreduced very sharply.; A r When the circuit is suitably'proportioned,the

V shunting eifect of Cg and the current resonating V eifect of C'increase at approximately the same rate, so that asthe'load'impedancedecreases with increasing frequency,'the currentthrough it increases-.atflthe. proper rate to keepthe :voltage acrossthe load impedance substantially constant. Suitable proportions include,among a other things, theright amount of damping resistance 2l, asthisresistance controls theamount of circulatingcurrent, andthe rate atwhich this va-.

frequency of C and coil I 9', since; Cg is shunted by "thelload resistor23, which is a low resistance shunting it.

ries withfrequency; If thisresistor is too small, the currentthrough theload resistor will'rise to a highiv'alue at: the resonantfrequency, andthe frequencyresponse curve Willhave a peak at this 1 frequency as showninFig.,.3. v The capacitygC has no effect on, the resonant relativetothe'reactance of C At the same time,

Iresistor'23 provides a'comparativelysmall amount of damping for theresonant circuit when the tap i=39'is inits maximumg'ainposition'because of Cg As tap39 is moved .whereby the high frequencyresponse tends to improve; also, the portion of 23 above'thetap' becomesefi'ectiveas damping resistance and ,Without the plate inductancel9,}the capacity" 'C and the capacity Cg are in parallel, and in a shuntto the load resistance 23, causing the load impedancetodecrease at highfrequencies, andv If tube 1 is a screen grid tube, its plateimpedance isr V V amount of resistance across to reduce the gain, the shuntingactionof-Cg on resistor 23' is reduced 7 said 'resistorjthrough a variabletap.

hence'reduces the effectiveness of the resonant 7 action in increasingthe high frequency response,

these two eifects'working in opposite directions v maintain thefrequency characteristic substa'n-f tially, constant as theamplification is reduced.

In Figs. 1 and 2 like parts are indicated by the same referencenumerals. In thegamplifier shown in Fig. 2, three-element vacuum tubes43 and 45 are employed and the single inductance coil [9 V ,ofFig, 1 hasbeen split into two sections "and A9, for'the purpose of minimizing theeffect of distributedcapacity in the coil. Where threeelement vacuumtubes'ar'e employed it may be de-' sirable to omit the damping resistor2|, since 1 the plate impedance of tubes of this type is low, enough to.cause considerable damping in the i circuit;

-.On the drawing various circuit values have been indicated, by. way ofexample, in ohms, millihenrys, and .microfarads. stood'that thecircuitjvalues may be varied within rather wide limits, difierent valuesbeing desirable for different typesoi tubes.

From the foregoing'descriptio n it will be apparentthat various othermodifications may be made in my invention without departing from the.

It should be underspirit and scope thereof, and I desire, therefore,that only such limitations shall be placed thereon as arenecessitated'by the prior art .and'are-set forth in the appendedclaims.v

' I claim as my invention;

' 1.1-An "amplifier "comprising'am el'e t ztric dis charge" tube of thescreen grid type 'havingan output electrode which has a certain amountof capacity to ground, anoutput circuit connected to said electrode,said circuit including an induc 'tance coil adjacent to said electrodeand a resistor connected in series with said coil between L the coiland'a source of potential for saidielectrode, said coiljand saidcapacity ,being'resona'nt:

at the high frequency endof the amplifiers frequency range, a. secondelectric discharge tube having an input circuit, means for impressing atleast a portion of the voltage drop'across' said resistor upon'saidinputcircuihand adamping "resistor connected'in: series with saidcoiland ,isaid firstresist'or' whereby said damping resistor V is notshunted bythe input capacity of said second tube 2. 'Anamplifieraccordingto claim, licharacterized in thatsaid meansincludesa-variabletap on said resistor connected to' the 'grid of the .second' tubewhereby the gain of the amplifier may be controlled. 1

V 3. An amplifier comprising an, electric distWeen a-n output circuitconnected to said elecsistor connected in series, said coil havingan.in-'

ductance value such that it resonates withsaid capacity at the highfrequencyend of the amplifier s frequency range, ;a work circuit coupledacross said resistor only, and a damping resistor connected in serieswith said coil and said first resistor whereby, said damping resistor isnot shunted by said work circuit? 7 I '4. Apparatus according to claim 3charaeterized in that means is provided for varying the cuit'isconnected. r a '5. Apparatus according to claim 3 characwhich said-workcirterized inthat said worl circuit is connected to charge tube whichincludes output electrodes; T ihaving a certain amount of capacitytherebe'- charge tube which includes output electrodes havwith saidcapacity at the high frequency end of ing a certain amount of capacitytherebetween, the amplifiers frequency range, a work circuit, an outputcircuit connected to said electrodes and and means for coupling saidwork circuit across including an inductance coil, a damping resistorselected portions of said load resistance only.

5 and a load resistor connected in series, said coil having aninductance value such that it resonates RENE A. BRADEN.

